The present invention concerns an electrode. More particularly, the invention relates to an electrode for use in electro-optical devices, such as displays or solar cells.
Many different kinds of electrodes are known. Various electrodes for electro-optical applications and devices have been reported widely in recent years. Of particular interest is the electrical and optical characteristics of such electrodes.
U.S. Pat. No. 4,518,891 to Howard, Jr. discloses large area electroluminescent (EL) faceplates for a storage cathode ray tube (CRT). It describes a macroscopic metallic mesh to hold the electrical potential constant.
Another metal structure called metal assist structure is disclosed in U.S. Pat. No. 5,455,899 to Budzilek et al., which is related to electroluminescent devices in general.
U.S. Pat. No. 5,131,065 to Briggs et al. discloses an optically enhanced flat display panel. It describes narrow highly conductive strips to obtain low resistance.
U.S. Pat. No. 5,293,564 to Tadros et al. discloses an improved working electrode for use in display devices. This electrode has an oxide coated metal grid. The metal grid and coating are disposed on a transparent substrate which serves to provide structural integrity.
Nevertheless, all reports including the mentioned disclosures are paying for its better conductivity with a lower quality of its optical characteristics. Recently, transparent conducting oxide (TCO) thin films have been widely used as the transparent electrode for flat-panel displays and solar cells. However, the TCO films in practical use for these devices still have several unsolved problems such as high cost, low transparency, and poor conductivity. Highly conductive and transparent undoped or impurity-doped ZnO (Zinc Oxide) or doped AlO (Aluminum Oxide) thin films have recently gained much attention because ZnO is an inexpensive and abundant material. Another disadvantage of the state of the art is that usual electrodes are not absolutely invisible and cover a large portion of an electro-optical device.
An object of the present invention is to overcome the disadvantages of known approaches.
It is an object of the present invention to provide a new electrode for use in electro-optical devices.
It is generally an object of the present invention to improve the electrical and optical properties of an electrode in electro-optical devices.
It is a further object of the present invention to provide an electrode with high conductivity, high transparency and with a wide adaptability.
It is another object of the present invention to provide an electrode with an excellent conductivity-transparency ratio.
It is also an object of the present invention to provide an electrode with highly adaptable electrical, optical, electronic and interface characteristics.
Another object of the present invention is to provide an improved display device with the new electrode which is characterized by high transmittance and improved uniformity.
The present invention provides an electrode for an electro-optical device. Light is passing through the electrode and the electrode comprises a pattern of conductive elements. The elements have dimensions small compared to the wavelength xcex of light, which means the elements comprise a perimeter p smaller than the wavelength xcex of the passing light. The elements may have any kind of structure and the word perimeter herein used is meant to cover any kind of size or shape suited to design a transparent electrode.
The electrode is penetrable by light, comprises longitudinal electrically conductive elements and spaces, whereby the light intensity distribution after having penetrated the electrode compared with the light intensity distribution before having penetrated the electrode is influenced by dominant forward scattering. According to the present invention, more forward scattering than backward scattering is achievable, which means that the transmission loss is less than the surface coverage.
Due to the small geometry of the elements, which can be straps or spheres or combinations thereof, Mie scattering occurs, making the electrode appear transparent for a certain wavelength range. It is an advantage of the present invention that the transparency of such an electrode is very high, because some incidence of light on the small elements will be forward scattered and no longer reflected backwards as aforementioned. The total percentage of coverage caused by conductive elements is low and the power distribution is very high compared to today""s solutions. The present invention provides the ability to make displays and all other electro-optical devices with better performance. In the following the electrode according to the present invention will also be referred to as transparent electrode.
The elements comprise a suited material, such as metal, an alloy, semiconductor or a conductive polymer to guarantee for a good conductivity. It should be noted that the word electrode herein used is meant to cover the conductive elements and the spaces, because the spaces together with the conductive elements influence the electrode""s properties. Furthermore, the conductive are arranged in form of a regular pattern or structure and have a size and a geometry which might determine the direction of the light. The geometry of the electrode, that means the pattern of conductive elements is highly application specific. For instance, optical effects like polarization and viewing angle adjustments are achievable. An enhancement will be achieved if the electrode is designed appropriately. The redundancy of the structure of the elements, which will electrically compensate broken elements or straps, is a huge advantage for every production process.
The conductive elements of the inventive electrode has a surface coverage of about 1% to 20% and preferably less than 11% or even less than 6%. Because of the forward scattered light, the loss of light will be less than the values mentioned before. This will provide more transparency compared to common electrodes. Less energy is necessary and therefore portable battery-powered applications can be used for a longer time.
A resistance of about 0.01 xcexa9/xe2x96xa1 to 100 xcexa9/xe2x96xa1, preferably less than 1 xcexa9/xe2x96xa1, can be achieved. For purposes of the present invention the term xe2x80x9cohms per square (xcexa9/xe2x96xa1)xe2x80x9d is used to designate sheet resistivity over a given surface area in a conventional manner.
The electrode has a topside modifier and a backside modifier. In the following, topside modifier and backside modifier together will be called modifiers. The modifiers are optional. The modifiers can be built of any suited material. Their structure depends on the application. It is an advantage of the electrode according to the present invention that modifier(s) can be employed. For example
the modifiers are made of SiOx which can have protecting functions; or
the modifiers itself may be bragg-mirrors to enhance optical characteristics; or
the modifiers can have the function of injection electrodes to inject excitons into a semiconductor; or
the modifiers can act as a unification layer to planarize the surface of the electrode; or
the modifiers can be used to build cavities into the structure openings.
The electrode can be manufactured using for example one of the following techniques: the so called microcontact printing or processing, photolithography, thermal evaporation, sputtering, coating and etching techniques. Submicron lithography can be achieved by the known VIS/UV light lithography, by X-ray lithography and e-beam lithography, and by scanning probe techniques and related techniques. A general advantage of the transparent electrode is the possibility of its preprocessability without any application specific structures, which is a result of the structure of the transparent electrode. This brings the advantage of less expensive processing steps and the possibility to manufacture the transparent electrode as a semi-finished product for example on glass or synthetic materials.
The transparent electrode is particularly suited for electro-optical applications and devices. The excellent conductivity-transparency ratio has its roots in the small dimensions of the conducting structure, which can be a regular grid. The perimeter of the conductive elements are small compared to the wavelength of the light. Thus, the optical distortions can be minimized and the electrode itself gets invisible and transparent. Moreover, the small dimensions cause into partial forward scattering of light. The forward scattering of light leads to a higher transmission rate of the transparent electrode.
The electrode according to the present invention is usually part of an electro-optical device that emits light, as there are: lasers, lamps, cell phones, pagers, alarm clocks, digital watches, radios, personal digital assistants, television sets or flat panel TV displays, information display terminals, flat-panel displays and any kind of displays or devices with an organic (polymer and molecular) light emitting diode (OLED) technology, e.g. for use in notebook displays, shop windows, car windows or car displays, toys, credit cards, small cellular phones, large flat screen high definition televisions and so on. Organic electroluminescent device and OLED technology has been determined to potentially provide outstanding display performance through high light emitting efficiency, which means high brightness and excellent luminosity, high resolution, full color capability, low power and long operating life.
The electrode which is electrically conductive and optically transparent may substitute indium-tin-oxide (ITO) for transparent electrodes in electro-optical devices.
Furthermore, the electrode according to the present invention is a conductor or a conductor element, which means that the transparent electrode is executed as a lead. This might be an invisible lead to a device in or on a shop window or a car window, for instance, for an alarm signal which is displayed on the window. However, the transparent electrode may be used as a conductor or as a conductor element wherever an invisible lead or conductor on a transparent medium is required or desired.
The electrode is not only suited for electro-optical applications but also for optical applications and devices. A window or a glass front of buildings or offices with such a transparent electrode makes it more bright in the inside. As part of a light emitting device, the transparent electrode provides a higher-light efficiency and helps therefore to save energy.
The electrode according to the present invention might also be part of an electro-optical device that receives light. This might be solar cells or photodetectors, like photodiodes, photoresistors, and phototransistors. The optical transmission coefficient will be improved for such devices and the internal resistance becomes smaller. With the present invention the efficiency of solar cells and other devices will be enhanced dramatically.